The contact pathway of blood clotting contributes to thrombotic diseases but either has no impact on hemostasis or is associated with relatively mild bleeding diatheses in man. Furthermore, the contact pathway contributes to inflammation, kinin generation, and complement, placing it at the nexus of thrombotic and inflammatory pathways. This raises the possibility that interrupting the contact pathway might be strongly antithrombotic/anti-inflammatory with fewer bleeding side effects compared to anticoagulants that target the common pathway of clotting. Work from our lab has shown that inorganic polyphosphate, which is secreted by activated platelets and which is present in infectious microorganisms, is highly procoagulant and is likely one of the long-sought pathophysiologic activators of the contact pathway. Subsequent work from our lab and others has now demonstrated polyphosphate to be an important link between hemostasis and inflammation/innate immunity. In the present grant period, we have further increased the knowledge of how polyphosphate regulates the blood clotting system. In particular, we showed that polyphosphate is a highly active cofactor for factor XI function, which may explain its otherwise puzzling contribution to hemostasis. We identified a series of new polyphosphate inhibitors that protect against thrombosis in vivo, and we also showed that polyphosphate modulates the complement cascade. This growing body of work demonstrates polyphosphate to be a regulatory molecule at the nexus of hemostasis, thrombosis and inflammation/innate immunity. Although much has been learned about the contributions of polyphosphate to these processes, much remains to be understood, and this is the focus of this renewal application. In the current application, we will address three specific aims focusing on: (1) Understanding how polyphosphate's procoagulant activities are regulated by metal ions, comparing the relative procoagulant activities of polyphosphate and extracellular nucleic acids, and understanding how polyphosphate and extracellular histones work together to activate clotting; (2) Understanding the mechanism(s) that control decay of polyphosphate in blood; and (3) Developing effective, highly biocompatible polyphosphate inhibitors as novel, safe antithrombotic agents.